The 2016 season is barely into its second week yet the game's top female players are falling like flies in the Australian outback.

Petra Kvitova and Agnieszka Radwanska, the top two seeds in this week's Sydney International, both withdrew from the event yesterday, citing a stomach virus and a leg injury respectively.

In the opening week of the season the world's top four players all retired or withdrew from tournaments.

Serena Williams suffered inflammation in her left knee at the Hopman Cup in Perth, while Simona Halep (ankle), Garbine Muguruza (foot) and Maria Sharapova (arm) all made premature departures from Brisbane, where Victoria Azarenka beat Angelique Kerber 6-3, 6-1 in Saturday's final to claim her first title for more than two years.

Kvitova's health is of particular concern after the two-times Wimbledon champion retired for the same reason from her first-round match last week at the Shenzhen Open in China, where Radwanska went on to win the title. "I'm really disappointed to have to withdraw but unfortunately I am still not feeling well," Kvitova said.

Sloane Stephens, who won the title in Auckland on Saturday, joined the sick list when she withdrew from this week's Hobart International, where Britain's Heather Watson, the defending champion, and Johanna Konta were due to play their opening matches today.

The first weekend of the men's season produced some familiar names among the finalists. Novak Djokovic crushed Rafael Nadal 6-1, 6-2 in the final of the Qatar Open, which was the only tournament last year at which the world No 1 failed to make the final, while Stan Wawrinka claimed his third successive Chennai Open title by beating Borna Coric 6-3, 7-5.